Metallocene Catalyst and Process for Producing Olefin Polymer
Since Kaminski, Sinn, et al. discovered that a polymerization catalyst comprising a metallocene compound having a ligand having two cyclopentadienyl ring structures and a methylalumoxane synthesized from trimethylaluminum and water, is capable of polymerizing ethylene with high activity (Angew, Chem., Int. Ed. Engl., 19, 390 (1980)), olefin polymerization by means of a so-called single-site polymerization catalyst has been actively researched and practically used.
Particularly, olefin polymerization catalysts constituted by transition metal compounds activated by a cocatalyst comprising a methylalumoxane or a boron compound, have been widely used as catalysts for producing polyolefins. Among them, polymerization catalysts containing a metallocene compound having a structure in which two ligands are crosslinked by carbon or silicon, are known particularly as catalysts for producing LLDPE, or isotactic or syndiotactic polypropylenes.
However, they have a problem that the methylalumoxane or the boron compound to be used as a cocatalyst, is expensive.
JP-A-3-197513 discloses a polymerization catalyst constituted by an alkyl aluminum and a certain specific transition metal compound (a metallocene compound) without using the methyl alumoxane or the boron compound as a cocatalyst. However, with the transition metal compound employed, the activity is low, and the molecular weight of the obtainable polymer is low, whereby it is still poor in practical applicability. Even if it is attempted to improve the activity at a higher polymerization temperature, it is considered that at a polymerization temperature condition higher than the polymerization temperature of 50° C. specifically disclosed, the molecular weight tends to decrease, such being poor in practical applicability.
Further, JP-A-7-62012 discloses a method for polymerization under such a condition that the aluminum/transition metal ratio of a transition metal compound and an alkyl aluminum compound is from 1 to 500 by molar ratio calculated as atoms. However, by such a ratio and with such a transition metal compound employed, it is considered that the activity is low, and such is poor in practical applicability. Further, with the transition metal compound employed, even if it is attempted to improve the activity at a higher polymerization temperature, it is considered that at a polymerization temperature condition higher than the polymerization temperature specifically disclosed, the molecular weight tends to be low, such being poor in practical applicability.
It is an object of the present invention to provide a process for producing an olefin polymer with higher polymerization activity without using an expensive co-catalyst or using such a cocatalyst within a limited range, more particularly a process for producing a high molecular weight (co)polymer with high polymerization activity even at a high polymerization temperature which is more practical.